LAUSR

Stroboscopic time-resolved TEM capable of reaching temporal resolutions that go beyond detector limits has been in use for decades (see, for example, [1]). With few exceptions, this early work, and the more recent work focused on studying materials processes that form new structures or states without returning to the initial conditions on typical experimental timescales [2], probed mainly nanosecond dynamics. Again with few exceptions, the emphasis on going beyond nanoseconds to the sub-picosecond regime in the TEM began in earnest relatively recently with developments employing femtosecond (fs) pulsed lasers and commercial conventional instruments, principles of ultrafast pump-and-probe spectroscopy and scattering, and general concepts of stroboscopic imaging [3,4]. Though a variety of source types are now in use [5-7], the basic approach to reaching sub-picosecond (i.e., hundreds of femtoseconds) time resolution in what is called ultrafast electron microscopy (UEM) is generally the same. In short, this involves limiting the number of electrons in each packet in order to preserve coherence and thus reach high spatial and temporal resolutions. Accordingly, this approach requires repeated pumpand-probe cycles at a fixed time point in order to accumulate signal, thus necessitating specimen relaxation to the pre pump-pulse condition before arrival of the next excitation pulse. While seemingly complex, numerous novel studies have been successfully conducted in this way [8-10], and the approach is now reaching a level of establishment that makes reflection upon current state-of-the-art capabilities as well as speculation on future advances timely.